This invention relates to modulator circuits and more particularly to a self balancing modulator for eliminating direct current offset errors at the output thereof when the modulator is in a balanced operating mode.
Modulator circuits for modulating a subcarrier frequency by an information containing signal are well known in the art. One type of prior art modulator comprises a differential input amplifier adapted to receive the information carrying input signal with differential outputs thereof coupled respectively to a pair of differentially connected transistor pairs having their collectors interconnected such that in response to the subcarrier frequency or switching signal applied to the bases of the respective transistor pairs there appears at the output of the modulator the subcarrier frequency amplitude modulated by the information carrying input signal.
A specific application utilizing a modulator of the type aforedescribed is PAL-Secam Television Chroma Decoding Systems in which the Secam chroma information is remodulated onto a Pal frequency or other subcarrier frequency before being applied to a commutator. It is well known to those in the art that in typical Secam television receiver systems the Secam chroma information is generally applied to a frequency discriminator at the output of which appears a demodulated Secam chroma information signal which must be synchronized in accordance with standard techniques. In order to effect this synchronization alternate frequency sequences are produced by supplying the output of the aforementioned frequency discriminator directly and indirectly through a delay line as is understood. Typical delay lines used in contemporary Secam television chroma decoding systems require that the Secam chroma information signal be used to remodulate a subcarrier frequency before being applied to the delay line. The remodulator must be extremely well balanced so that the residual levels of the subcarrier are very low: a residual level represents a black level error. A manual adjustment of the modulator to reduce residual levels is not desirable in television receiver systems and therefore some automatic balancing techniques must be applied. Thus, there exists a need to provide a self balancing modulator such that with the Secam chroma information being at a zero level the output of the modulator will be substantially zero in order to prohibit any output offset errors.